1. Field of the Invention
The present invention relates to a semiconductor laser device which can generate an optical higher harmonic wave.
2. Description of the Prior Art
Many workers have tried to generate an optical higher harmonic wave by using nonlinear optical material in order to convert infrared light to visible light. A higher harmonic wave can be generated by making use fo the nonlinearity of the refractive index and polarizability of the optical material. Unfortunately, the conversion ratio is very small because the nonlinear constants of a nonlinear optical material are very small.
The conversion efficiency .eta. from a fundamental wave to a second harmonic wave can approximately be given as follows: ##EQU1## where .DELTA.K is a difference between the wave number of the second harmonic wave and that of the fundamental wave, l and d are the length and the width of a nonlinear medium (optical waveguide), respectively, A is a sectional area of the optical waveguide, and P is the power of the incident fundamental wave.
The formula shows that the enhancement of conversion efficiency .eta. needs an increase in the length l of nonlinear medium (optical waveguide), an increase in the power density P.omega./A of the incident fundamental wave, and/or a phase matching between the fundamental wave and the second harmonic wave in order to decrease the difference .DELTA.k.multidot.l in phase to zero. For example, .eta. becomes 1.5% if lithium niobate (LiNbO.sub.3) is used for the nonlinear optical material and the incident fundamental wave of 10.sup.7 W/cm.sup.2 of P.omega./A (which is equivalent to the case that an incident fundamental wave of 100 mW is introduced into on an optical waveguide having a sectional area of 1 .parallel.m.times.1 .mu.m) is guided in an optical waveguide of 1 mm of length.
As for the enhancement due to an increase in P.omega./A, Uesugi et al. (Oyo Butsuri, Vol. 49, No. 12 (1980) P. 1234-1238 (in Japanese)) obtained a high conversion efficiency of 0.77% based upon an experiment for generating a second harmonic wave in use of a three-dimensional optical waveguide of LiNbO.sub.3. However, their method requires a very high precision in arranging crystal axes and in controlling the temperature because the phase matching is obtained in such a manner that the refractive index of ordinary rays and that of extraordinary rays are made equal by making use of temperature dependences of these refractive indexes. Therefore, it is very difficult to obtain a high conversion efficiency. Then, they tried to increase the conversion efficiency by making the length l of the optical waveguide a few cenimeters longer which is one or more orders of magnitude longer than that of an ordinary optical waveguide.